This invention relates to a photoconductive member which exhibits photoconductivity upon illumination with electromagnetic light in the infrared, visible, ultra-violet, X-ray, and .gamma.-ray regions, and which facilitates formation of a latent electrostatic image.
In an image formation technique related to electro-photographic photosensitive members, solid-state imaging elements and the like, photoconductive members exhibiting photoconductivity are used. Such photoconductive members must have high resistivity in dark conditions (normally, 10.sup.13 .OMEGA..cm or higher) and low resistivity in bright conditions.
In an electrophotographic photosensitive member, electric charges are uniformly charged on a body surface by corona discharge. Then, light corresponding to an original pattern is radiated on the surface of the photosensitive member. At the position irradiated with light, pairs of electrons and holes are generated inside the photosensitive member. Either electrons or holes are combined with the electric charges (positive or negative charges) on the surface of the photosensitive member that is to be neutralized. The remaining holes or electrons propagate through a photoconductive layer and reach a conductive substrate. Thus, a latent electrostatic image is formed on the surface of the photosensitive member by electric charges which are not neutralized. Toner (black powder) charged to a polarity opposite that of the charges on the surface of the photosensitive member is supplied onto the surface thereof and is attracted by Coulomb attraction so as to form a toner image. In this case, the potential of the developing unit is set slightly higher than that of the photosensitive member so that an electric field opposite to that of the charges is formed between the photosensitive member and the developing unit and a developing bias is applied thereto. For this reason, if the surface of the photosensitive member is not charged, the toner cannot be attracted to the photosensitive member by the charge of the toner itself.
The following characteristics are required of a photosensitive member.
(1) Electric charges on the surface of the photosensitive member must be carried on the surface until light is radiated.
(2) One of a paired electron and hole neutralizes the electric charge on the surface of the photosensitive member without recombination, and the remaining hole or electron reaches the conductive substrate of the photosensitive member in a short period of time.
In the electrophotographic photosensitive member having such requirements, an amorphous chalcogenide material is conventionally used for a photoconductive member. The photoconductive member formed of the material can be fabricated to have a large area. However, since the material absorbs light varying from visible light to ultraviolet light, photosensitivity in the visible light region is practically low. Because amorphous chalcogenide is insufficiently hard, it has a short lifetime.
Recently amorphous silicon (to be referred to as a-Si hereinafter) has been attracting much attention as a photoconductive material. The a-Si has a wide absorption wave range, is panchromatic, and is very hard. When the a-Si is applied to a photosensitive member it has a lifetime 10 times that of a conventional photosensitive member. Furthermore, the a-Si is not toxic. In addition, since the a-Si is inexpensive, a photosensitive member of a large area can be easily achieved as compared to one composed of, say, single crystalline silicon.
However, since the a-Si normally has a resistivity of about 10.sup.8 to 10.sup.10 .OMEGA..cm in a dark condition (can simply be called a "dark resistivity"), electric charges on a surface of a photosensitive member where a latent image is to be formed, cannot be held. In a photosensitive member an attemp is made such that an insulative layer (blocking layer) having a high resistivity such as silicon oxide, silicon carbide, silicon nitride and the like is interposed between the a-Si photoconductive layer and a substrate so that carriers are prevented from being injected into the photoconductive layer from the conductive substrate. In this case, however, when the thickness of the high resistivity (blocking layer) insulative layer is increased, transmission of carriers from the a-Si layer, formed on the insulative layer, to the conductive substrate is interrupted, thus causing a residual potential. On the other hand, when the thickness of the insulative layer is decreased, a sufficient potential holding function cannot be provided.
Meanwhile, a method has been proposed in which a semiconductor film having a p- or n-type conductivity is interposed between a conductive substrate and a photoconductive layer. Normally, a-Si in which B (boron) or P (phosphorus) is doped to a high concentration is used for this purpose. Such a layer is called a blocking layer. The charge blocking property of the blocking layer can be improved by doping it with a high concentration of B or P. However, such a layer has a high degree of internal stress. When a film having a different level of stress is stacked thereon, the film cannot be held and is easily removed.
Light absorption of a-Si occurs over a wide wavelength range, and absorbency is only gradually decreased, even near the absorption edge. That is, although absorbency is decreased within a wavelength range between 700 nm and 800 nm, it is not decreased to 0, thus permitting slight absorption of light. Therefore, when a photoconductive layer is formed of such a material and has so great a film thickness, as that found in a photosensitive member, light having a long wavelength may be absorbed by a portion near the base of the photoconductive layer. Since the electrons and holes of a-Si have low mobility, carriers generated away from the surface of the photosensitive member tend to remain in the a-Si layer. An electrophotography apparatus has a so-called discharging process wherein residual charges on the surface of the photosensitive member are erased after a transfer of an image is performed. When this process is performed by exposure, residual carriers inside the a-Si film neutralize surface charges formed on the photosensitive member in preparation for obtaining the next image. Therefore, the charging property after exposure is considerably degraded as compared to that in dark conditions.